In the related art, there is known an IP (Internet Protocol) network as a communication network. As a communication device of such a network, there is known a communication device (such as a router) having a function to discard a predetermined packets among received packets to prevent it from being transmitted downstream (for example, see JP-A-2001-36561).
According to such a communication device having a filtering function, it is possible to prevent an unauthorized access from outside thus enhancing the security in the network.